Currently, most electricity is generated by large power stations using fuel such as natural gas or coal. In this traditional method of electricity generation, approximately one third of the energy from the fuel is converted to electricity, while approximately two thirds is converted into heat. This heat energy, while potentially useful, is difficult to sell to consumers, as this heat is difficult to transport and potential users are generally some distance away. The heat generated is thus often not used and is surrendered to the atmosphere through cooling towers or through cooling water from the sea.
At the same time, potential users of heat including factories, hospitals and heated swimming pools often then generate their own heat by burning more fuel or by the use of electricity. This is clearly a highly inefficient method of generating heat and electricity and it has been found that it is far more cost effective and efficient for such users to install their own small “power stations”, such that they can use the waste heat generated from the electricity generating process directly, to meet their own heating requirements.
The simultaneous production of usable heat and electricity in the same plant is known as cogeneration. Cogeneration units are used in many of the industrial applications described above. However, the concept is equally applicable to domestic environments, such as households that require heating for hot water, and also for general heating of the environment, particularly in colder climates.
The benefits of cogeneration for domestic or household use can be enhanced if surplus electricity generated from such units can be provided to the mains distribution network for use by other households or commercial users.
Furthermore, mains connection allows power for peak household loads to be drawn from the mains distribution network. The cogeneration unit then does not need to be sized to meet these peak loads. A number of methods have previously been proposed to achieve this. However, current methods are generally very expensive and are rather complex to design and install.
The expense and complexity of such systems may be attributed to an inverter, which, is often used with small scale power generators to provide a convenient interface to the mains distribution network. In systems of this type, AC electricity generated by an alternator is rectified to DC and fed into the inverter. The inverter synthesizes AC power which exactly matches mains parameters (for example, frequency and voltage) and at the desired power factor. An inverter of this type typically senses the mains parameters and synthesizes the AC power to suit.
It is an aim of the present invention to provide a control system for use with a cogeneration unit that can be connected to a mains power distribution network in a more simple and cost effective manner.
It is also an aim of the present invention to provide a control system for use with a cogeneration unit, which unit is able to operate independently of an external AC power source (such as, the mains supply from a mains power distribution network).